1. According to the causes of failure, there are inherent failures of CNC machine tools and failures external to CNC machine tools.
(1) The inherent failures of CNC machine tools are caused by factors internal to the machine tool itself and are not related to external environmental conditions. The vast majority of CNC machine tool failures fall into this category.
(2) External failures of CNC machine tools are caused by external factors. These include low or high voltage fluctuations, incorrect power supply phase sequence or unbalanced three-phase input voltage, high ambient temperature, harmful gases, humidity, dust, external vibrations and interference, etc.
(3) Human factors are also one of the external causes of failures in CNC machine tools. During the first year of use, improper operation by unskilled workers causes more than one-third of total machine tool failures.
2. Common faults of CNC machine tools and treatment methods
(I) Collision accidents with machine tools.
When encountering this problem, first secure the scene, understand what state the machine tool was in when the failure occurred, distinguish whether it was the first processing or in the middle of processing, and the status of the operator at the time.
The main reasons for these problems are as follows: employees forget to return to the reference point before the first processing, or although the machine tool returns to the reference point, the operator does not pay attention to incorrect operations. Another cause is incorrect data entry when modifying the program. Some operators are careless and install the part backwards, causing a collision.
(II) Processing dimensions exceed tolerances.
There are many factors that cause the machine tool size to exceed tolerances.
When machining the surface size, geometric shape and relative position of the system links between them are changed at any time, the consequences will be obvious on the part, causing size fluctuations.
The following presents in detail the oversize failure caused by the transmission gap between the X and Z direction drive systems of the CNC machine tool.
In general, the sequence of operation is first electrical and then mechanical. First, measure the transmission clearance of X axis and Z axis. Typically, X axis ≤ 0.005mm, Z axis ≤ 0.01mm.
If it exceeds the above standard value, it means that the transmission clearance of the X (Z) axis is too large, which is why the part dimension is oversized. The solution to this problem is to perform backlash compensation in the system environment.
For FANUC system, set it to N 00N00; for the Mori Seiki II NC system, set it to N0000 N000 and be sure to disconnect the power before setting. The limit of this compensation value is within the range of (0.5 ~ 0.8), beyond which there will be danger.
If the transmission backlash is too large, a mechanical backlash adjustment must be performed. First adjust the transmission clearance between the ball screw and the servo motor. The equipment adjustment method varies due to different equipment and transmission methods.
At this time, you can refer to the random instruction manual. Then adjust the ball screw installation bearing clearance, and the adjustment degree should be flexible and uniform throughout the entire stroke without damping.
After these adjustments, it is usually necessary to reset the backlash compensation as described above.
( III ) CNC lathe failures
It can be said that the frequency of use of the CNC lathe is incomparable to other components of the CNC machine.
Therefore, due to its poor working environment and complex internal structure, the probability of failure is particularly high.
Phenomenon 1:
The tool holder does not rotate the position (the system often displays a tool holder position signal error), and there are many reasons that may cause the tool holder not to rotate the position.
Cause analysis:
After electrical overload, the tool holder will pop out automatically. The 380V phase error of the tool holder, because the tool holder can only rotate clockwise when rotating (there is a directional positioning mechanism inside the tool holder), so once the three-phase power phase is connected incorrectly, the electric tool holder motor will reverse after being turned on and the tool holder will not be able to rotate; the three-phase power supply for the tool holder electric motor is missing and the 24 V power supply used for the tool holder position signal is defective.
The thrust positioning plate crushed the thrust ball bearing on the central shaft inside the tool holder body, making the bearing unable to rotate, and the tool holder's electric motor was unable to drive the tool holder to rotate.
After removing the parts, it was found that the screws were loose. This is because the vibration caused by tool holder turning brings long-term tangential forces in positive and negative directions to the positioning wrench, causing damage to the positioning wrench.
The positioning plate and nut move downwards, exerting a greater axial force on the bearing, making it impossible to rotate.
For the failure of the “system locating board” in system control, after the tool holder is in place, the “system locating board” must be able to detect the position signal of the tool holder.
The measures we can take for the above reasons are: replace damaged parts, check the 24V power supply, check the strong power circuit of the tool holder, disassemble the tool holder, adjust the axial clearance of the thrust bearing, replace the “system location plate”, etc.
( 4 ) Electrical faults
(1) Reference point failure.
The failure of the machine tool to return to the reference point can generally be divided into two types: unable to find (deviate) the reference point and unable to find the reference point.
The first is mainly caused by improper setting of the position of the reference point switching block and only needs to be readjusted.
The accessory factory generally likes to use economical CNC lathes, although they are cheap, their protection measures are not very ideal, so the phenomenon of circuit break and short circuit caused by the travel switch input is common.
This last type of failure is caused by the invalidation of the zero marking pulse signal (including no signal generated or lost in transmission and processing) or by the signal generated by the deceleration switch when returning to the reference point.
To eliminate the fault, it is necessary to first understand the return mode of the machine tool to the reference point, and then carry out fault comparison analysis. The method we can adopt is to use “external” and “internal” methods and signal tracking to find the defective part.
Here, “internal” refers to the zero marking position on the grid ruler or the zero marking position of the pulse encoder.
Detection of the zero marking pulse signal can be checked with an oscilloscope; “external” refers to the reference point block and switch installed outside the machine tool, which can be directly observed for the presence or absence of signals using the I/O status indication of the CNC system PLC interface.
(2) Overtravel:
When the forward motion exceeds the hard limit set by the soft/hard limit switch or the soft limit set by the software, an overshoot alarm will occur. In this case, the fault can be eliminated and the alarm released according to the instructions in the CNC system manual.
(V) Incorrect tool parameters
In the lathe manufacturing process, the key to improving processing efficiency on a CNC lathe lies in correcting the parameters of the tool used.
If the tool parameters are used reasonably, it can not only improve the tool life, but also improve the processing efficiency and quality.
If tool parameters are used incorrectly, it will not only seriously affect the quality of the workpiece, but also require operators to constantly change, sharpen and align tools, making the CNC lathe unable to work continuously, which has a direct impact on production efficiency. At the same time, costs and profits will also be greatly reduced.
Therefore, the correct use of tools and tool parameters is very important for lathe processing. Tool parameters should be selected based on specific lathes, specific tools and specific processed materials.
Often, the maximum cutting rate of tool parameters should be selected based on meeting the requirements of machining equipment, which is beneficial to improving work efficiency.
Generally, people calculate the maximum and most suitable tool parameters or use reasonable mathematical models to test the best tool parameters.
At the same time, due to the limited types of tools, several commonly used tools can basically complete more than 80% of the total processing volume.
Therefore, we can choose reasonable tools based on the characteristics of processed materials in a small part of the workload and obtain the ideal cutting parameters of the tool in actual operation.
3. Path analysis of good operating habits for CNC lathe equipment
(1) Strictly follow the basic operating technology procedures of CNC lathe equipment and carry out the technical operation process of specific equipment production and processing links.
Ensure the reasonableness of the preparation of machine tool operators and, based on this, timely clean the environmental space of the production of mechanical components and maintain work.
(2) Before starting actual production activities on CNC lathe equipment, a detailed inspection and confirmation of the technical performance status of CNC lathe equipment must be carried out in advance to ensure that the actual machine tool equipment can maintain a Good technical performance and stable status during the actual production and processing activities of mechanical components.
(3) In the process of using CNC lathes to carry out mechanical component processing activities, according to the specific type of mechanical parts actually processed and the characteristics of the processing technology, targeted configuration of the operating parameters of the processing equipment must be made. CNC lathe.
Timely adjust the operating system parameters of the CNC lathe equipment to ensure that the CNC lathe equipment can maintain a stable and optimal performance status during the production and processing activities of mechanical components.
4. Conclusion
With the continuous progress of science and technology, the application of CNC lathes will become more and more widespread.
We analyze common problems with CNC lathes, find the causes of the problems and study methods to solve them.
We must develop good operating habits for CNC lathe equipment and continuously accumulate experience to produce higher quality products.
